As is well-understood in the art, periodic scatterometry targets are used to obtain accurate measurements of target features. Such targets include massive arrays of uniformly constructed and uniformly spaced periodic features arranged to provide the best possible targeting information. Typical prior art example targets include periodic gratings or periodically configured higher dimensional target arrays comprised of a plurality of uniformly spaced and sized metrology features.
Such periodic targeting structures typically feature two layers of similarly oriented periodic gratings formed one over the other. Typically, the layers are designed with a specified predetermined offset with respect to each other. This enables scattering signals to be generated when illuminated by a light beam. A comparison of the actual signal produced with the expected scattering signal enables highly accurate overlay metrology measurements to be made.
Generally, several different targets sequentially illuminated and measurements of the scattering signals are then used to make overlay measurements. Typically, the several targets each having different offsets which enable accurate overlay measurements to be made. These measurements enable a determination of alignment accuracy to be obtained for the various fabrication processes used to form the layers of a semiconductor wafer.
In general, prior art targeting arrays employ several different targets having a range of offsets (offsets between the top and bottom layer gratings of the target) to enable accurate overlay measurements to be made. Typical targeting arrays include a plurality of scatterometry targets arranged in complementary target pairs. A complementary target pair is a pair of targets that have an offset between gratings of a first amount (say an offset of “x” Ångstroms (Å) in a “positive” direction and a complementary offset of the same distance (say an offset of “−x” Å) in an opposite direction (i.e. a “negative” offset) to form a complementary pair of targets. Such a target is said to have a symmetrical scatterometry overlay (SCOL) offset.
Examples of prior art systems which rely on scatterometry techniques can be found in U.S. Pat. Nos. 5,867,276; 5,963,329; and 5,739,909. These patents describe using both spectrophotometry and spectroscopic ellipsometry to analyze periodic structures and are incorporated herein by reference. Another useful background reference describing many such scatterometry approaches is disclosed in the U.S. patent application Ser. No. 11/525,320 entitled “Apparatus and Methods for Detecting Overlay Errors Using Scatterometry” also incorporated by reference herein. Numerous other related approaches are also well known in the art.
FIG. 1(a) is a simplified diagram illustrating a commonly known targeting array 100. In most targeting arrangements known today these complementary target pairs are arranged vertically or horizontally adjacent pairs. The depicted illustration includes five adjacent complementary target pairs (101a, 101b, 101c, 101d, and 101e) arranged in a series of rows where each target in a complementary target pair is horizontally adjacent to the other target of the pair. Each target of the pair has gratings that are arranged parallel to the gratings of the other target in the pair. As mentioned above, each target in the complementary pair features a predetermined positive and negative offset. In FIG. 1(c), for example, targets 101a′ and 101a″ represent “x targets” (having gratings arranged parallel to the x-axis). Correspondingly, for example, FIG. 1(a) targets 101b′ and 101b″ represent “y targets” (having gratings arranged parallel to the y-axis).
FIG. 1(b) provides an illustration of a common illumination approach used in conjunction with the targeting arrangement depicted in FIG. 1(a). An illumination beam is directed onto on of the targets of the first complementary pair 101a to form an illumination spot 110 which is then moved to each target (e.g., following path 111) in the targeting array 100 to generate scattering signals that are collected and analyzed to generate overlay metrology measurements.
One unfortunate limitation of such a targeting arrangement deals with the fact that the illumination spot 110 is actually an Airy disk having portions of the optical signal that extend beyond the boundaries of each target and have the potential to generate large amounts of signal “contamination” by illuminating considerable portions of nearby complementary targets. Such signal contamination occurs when the optical signal of the illumination spot 110 extends onto the adjacent target of the complementary pair thereby generating scattering signal from the adjacent target. Such “cross-talk” can seriously degrade the fidelity and information content of the scattering signal. This is a serious problem that will be discussed in greater detail below.
An additional limitation of this existing approach is that it is slow. A single spot must be directed to each target on a targeting arrangement and then to each target on the entire wafer (there can be 100's or 1000's of such targets). Thus, it can take a considerable time to inspect an entire wafer.
Therefore, although such existing processes and tools are suitable for their intended purposes, improvements can be made. The present invention seeks to go beyond the limitations and structural shortcomings of existing technologies.